1. Field Of The Invention
The present invention relates to linearly reciprocating electrodynamic motors and alternators, and more particularly to linearly reciprocating electrodynamic motors and alternators having non-axisymmetric, interdigitating interfaces between moving and fixed magnetic circuit elements.
2. Description of the Prior Art
It is known in the art that moving flux fields in proximity to iron laminations surrounded by conductive coils will induce an electric field in, and corresponding voltage across, the conductive coils as a result of flux variations within the laminations, such as occurs in what are commonly known as alternators. Similarly, it is known that varying electrical current in conducting coils can be used to produce varying flux fields that are in turn used to produce mechanical movement (such as rotation or linear motion) of an appropriate magnetic member in proximity to the varying flux fields, such as occurs in what are commonly known as motors.
Alternators and motors in which the principal motion is along a single axis or line often are referred to as "linear" alternators and motors. Various applications for such linear alternators and motors are known in the art.
One application for a linear alternator is to be coupled to the piston of a Stirling engine. It is known to provide for hermetic sealing of high-pressure Stirling engines by incorporating a power conversion device, such as a linear electric alternator driven by the engine piston, within the pressure vessel. Such linear alternators are directly actuated by the reciprocation of the piston in order to avoid any intermediate linkages and the attendant need for lubrication, since lubricants typically must not be used within the pressure vessel of a Stirling engine.
In the past, such linear alternators have been constructed in substantially axisymmetric form, such that their operation was independent of the angular position of the plunger. This has been required with the commonly-used fluid-film, typically gas, bearings between the reciprocating plunger/piston and fixed supports. Such bearings require extremely close clearance fits, which are difficult in any shape, but are most readily achieved in circular form. Such axisymmetry to the piston/plunger typically precludes any non-sliding connections, it being unrestrained except to one axis and thereby allowing movement such as rotation. Sliding contacts are to be avoided in most applications for such machines in that rubbing surfaces would tend to prematurely wear out, and the desired long service life would not be achieved.
Electrically, this tends to constrain the moving magnetic flux generators on the plunger to be constructed of permanent magnets, typically in the form of annular rings or the like, requiring no electrical energization to create the moving magnetic flux fields. In such prior machines, the plunger typically must be round in section, and as a result permanent magnets must be specially shaped to fit such circular shape. In addition, for a given stroke and magnetic strength, the perimeter of the magnet ring usually is the limiting feature dimension for determining output power, forcing machines of large power output to be large in diameter, to the detriment of mass and structural rigidity.
With reference to FIG. 1, a typical permanent magnet linear alternator/motor machine of conventional design will be described.
As shown in the cross-sectional view of FIG. 1, all components are substantially axisymmetric around the indicated center axis. Four rings of permanent magnet material 1a and 1b are axially arrayed in cylindrical plunger assembly 8. Plunger assembly 8 comprises magnet rings 1a, and 1b, spacer ring(s) 2 of material with low magnetic permeability, and piston 7. Magnet rings 1a and 1b are spaced in axially-adjacent pairs of opposite magnetic polarity as illustrated, and the axial extent of each ring is substantially equal to the reciprocation stroke of plunger assembly 8. At the mid-stroke position of plunger assembly 8, the magnet ring pairs 1a and 1b are centered in gaps in a toroidal flux path defined by inner and outer radial magnetically-permeable lamination plates 4' and 4, respectively. A coil of electrically-conductive wire 5 is wound inside magnetically-permeable plates 4 that comprise part of the toroidal flux circuit. Plunger assembly 8 is supported and positioned for reciprocation by a sliding bearing 9 in support frame 6.
In operation, plunger assembly 8 moves alternately to either end of its stroke, bringing alternately the upper and lower members of each magnet ring pair 1a/1b into alignment with the gap in the toroidal, magnetically-permeable path defined by inner and outer radial lamination plates 4' and 4. Coil 5 thus is alternately surrounded (and linked with) a reversing magnetic flux in lamination plates 4' and 4. This flux induces alternating voltage in coil 5 according to known electromagnetic principles. When an electrical load is attached to the ends of coil 5, a current is enabled to flow, producing power.
Alternatively, an alternating electric current can be passed through coil 5, inducing a reversing magnetic flux in laminations 4 and 4', thereby causing plunger magnet rings 1a/1b to align alternately with the lamination flux direction. Since axially adjacent magnet rings 1a and 1b are of opposite polarity, reciprocation of plunger assembly 8 results as rings la having polarity which reinforces the flux in laminations 4 and 4' alternately align and then are expelled in favor of opposite-polarity rings 1b when the electrically-induced flux reverses, and vice versa.
The end result of such prior art machines is, however, that, in the case of an alternator, the alternator and its bearings typically constitute more than half the size and weight, and up to half the cost of a Stirling engine and linear alternator assembly. Similar disadvantages result when such prior machines are configured to be operative as a linear motor.